Water Heater Demand Side Management System

ABSTRACT

A system for shifting energy demand from on-peak time windows to off-peak time windows by using hot water heater load shifting, while providing the end user with the level of service (i.e., availability of hot water) according to the user&#39;s customary use described by service quality criteria. The shift is accomplished by a controller located at the end user establishment and in communication with a central control server. The controller monitors local water heater upper and/or lower temperature and controls upper and/or lower water heater heating elements in accordance with a demand shift process commanded by the central control server. The controller may determine usage and remaining capacity for reporting back to the central control server. A volumetric capacity and usage determination is disclosed. The control server may select water heaters according to use patterns and/or measured capacity. One embodiment is adapted for use with existing water heaters without disrupting safety features of the existing water heater.

RELATED APPLICATIONS

This application is an application claiming the benefit under 35 USC119(e) of prior U.S. Provisional Application 61/077,235, titled “WaterHeater Demand Side Management System,” filed Jul. 1, 2008 by Harbin etal., which is hereby incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention pertains generally to the field of intelligentpower distribution grid technology, more particularly to the field ofautomatic load management to shift and reduce peak load requirements.

BACKGROUND OF THE INVENTION

Utility control of domestic water heaters as a means to shed and/orshift electrical load during peak demand periods has been available forseveral decades. However, the effectiveness of currently deployedsystems has been somewhat limited in accuracy, reliability and detail.Current systems are typically unidirectional, i.e. commands can be sentfrom the utility to the water heater to shut off supply power duringpeak load periods, but there is no upstream communications to verifythat the command was received and acted upon, nor how much load wasavoided. In fact, it is not unusual for power utilities to discover thatthe system at the customer premise has been defeated or has otherwisebecome non-operational. Additionally, even these legacy unidirectionalcontrol systems often require substantial capital investment to build upthe required RF infrastructure (radio towers and backhaul). In light ofthe limitations of current water heater control systems, power utilitieshave been able to offer their customers only relatively small economicincentives to sign up for load shedding plans which have resulted inslow adoption rates.

In recent years, most of the developed world has seen the widespreaddeployment of various Wide Area Networks (WANs), from digital cellularRF networks to fiber to the home (FTTH), DSL, and broadband over cable.These networks typically feature a relatively low data transport cost.At the same time that these low cost WANs have become available, theworld has seen dramatic increases in the cost of petro-fuels, concernsover the emission of CO₂, and a reluctance to accept the building of newgeneration facilities, resulting in a major problem with supply anddemand for energy. These colliding factors have created both a greatneed to shift utility loads to off peak hours whenever possible and theinfrastructure required to manage customer side loads. Shifting load toavoid utilizing inefficient and environmentally threatening energy isthe fastest path to delay the demand crossing the supply curve and toavoid the tremendous investment in new power generation facilities.Recent reports estimate the cost of a new nuclear plant to be between $9to $14 billion.

Thus, there is a need for a system to reduce peak load, thus allowingbetter use of existing power plants and minimizing the need for newpower plant construction.

BRIEF DESCRIPTION OF THE INVENTION

Briefly, the present invention pertains to a system for shifting energydemand from on-peak time windows to off-peak time windows by using hotwater heater load shifting, while providing the end user with the levelof service (i.e., availability of hot water) according to the user'scustomary use described by service quality criteria. The shift isaccomplished by a controller located at the end user establishment andin communication with a central control server. The controller monitorslocal water heater upper and/or lower temperature and controls upperand/or lower water heater heating elements in accordance with a demandshift process commanded by the central control server. The controllermay determine usage and remaining capacity for reporting back to thecentral control server. A volumetric capacity and usage determination isdisclosed. The control server may select water heaters according to usepatterns and/or measured capacity. One embodiment is adapted for usewith existing water heaters without disrupting safety features of theexisting water heater.

The controller may also monitor power quality and/or water heater usagefor reporting back to the load management server. In one aspect of theinvention, the local controller may separately control the upper heaterand lower heater elements to shift the demand load.

In one embodiment, the upper and lower temperatures are lowered during ademand peak.

In another embodiment, the lower temperature is lowered or turned offduring a demand peak.

In a further embodiment, the water temperature may be raised before ademand peak to shift demand earlier in time.

In one aspect of the invention, the time at which a temperature isresumed after a peak is randomly varied or varied among a group ofestablishments to prevent simultaneous resumption of demand fromnumerous units.

In a further aspect the controller communicates with the load managementserver and periodically receives a demand profile and algorithm whichmay be used autonomously in the event of communication outage or toreduce network traffic required to maintain control.

In one embodiment several control procedures may be pre-stored in thecontroller so that the load management server need only select a prestored control procedure.

In a further aspect of the invention, the load management server maydirect immediate turn off of any number of water heaters or portions ofwater heaters in response to an extreme or unexpected peak demand.

In a further aspect the controller may monitor and log power qualitydata including voltage, load, and outage information and communicatewith the utility server to deliver power quality log data periodicallyor upon command.

In a further aspect of the invention, the controller may monitor and logwater heater demand information such as demand and temperature by timeof day, and may communicate with the load management server to deliverwater heater usage pattern data periodically or upon command orinitiated by the water heater controller.

In a further aspect of the invention, a first portion of the system maybe located at the consumer power meter and a second portion may belocated with the water heater. The first portion and second portion maycommunicate by wireless, wired, or power wiring carrier techniques aswell as conventional direct control wiring.

In a further aspect of the invention, hot water usage in gallons of hotwater at a desired temperature is estimated based on measuring an uppertemperature and a lower temperature of a water heater and observing thepower required to maintain the temperature.

In a further aspect of the invention, hot water capacity of a customer'shot water heater is maintained at a predetermined minimum number ofgallons of hot water at a desired temperature as a function of expectedusage over time.

In a further aspect of the invention, the control server may reducedemand by turning down hot water heaters selected first from users withexpected demand being the farthest in the future, and further that theusers have stored capacity in excess of predicted demand for eachrespective user.

In a further aspect of the invention, demand reduction is determinedaccording to demand reduction need per grid route.

In a further aspect of the invention, hot water volumetric usage andremaining capacity of a water heater is estimated based on a model ofhot water vertical temperature profile as a function of usage and powerapplied to the water heater. Alternative measurements include directmeasurement of water flow and inlet and outlet temperatures.

In a further aspect of the invention, sufficient monitor information canbe collected to determine the effect of peak shift achieved by thesystem.

In a further aspect of the invention, the water heater controller may beinstalled to control an existing water heater without voiding themanufacturer's warranty, without invalidating UL or other safetyapprovals and without violating building codes.

In a further aspect of the invention, the water heater controller may besupplied with an installation kit for a standard water heater, theinstallation kit including temperature sensors and mounting hardware.

In a further aspect of the invention, a particular household may beclassified according to usage patterns and peak profile algorithms maybe selected or adapted according to the usage patterns.

In a further aspect of the invention, the controller may have a useroverride selectable function and a user override event may becommunicated to the utility.

These and further benefits and features of the present invention areherein described in detail with reference to exemplary embodiments inaccordance with the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 illustrates a system according to one embodiment of the presentdisclosure.

FIG. 2 is an exemplary block diagram of the collar unit of FIG. 1.

FIG. 3 is a block diagram of the water heater 101 and the controller 104according to one embodiment of the present disclosure.

FIG. 4 illustrates an exemplary water heater with a double throwthermostat switch.

FIG. 5 illustrates an alternative controller separately controlling eachheating element of a dual element water heater by operating one heaterat a time.

FIG. 6 illustrates an alternative controller separately controlling eachheating element of a dual element water heater by operating each heaterindependently of the other.

FIG. 7 illustrates an exemplary temperature control method forcontrolling an upper and lower heating element of a water heater.

FIG. 8 illustrates an exemplary multiple segment demand shifting controlsequence.

FIG. 9 is a notional depiction of a typical water heater verticaltemperature profile model as hot water is used from the tank.

DETAILED DESCRIPTION OF THE INVENTION

The system described here allows the power utility providers to shiftenergy demand due to hot water heating from on-peak time windows tooff-peak windows while providing the end user with the level of service(i.e., availability of hot water) to which they have become accustomed.This shift of demand from on-peak time windows has the effect ofreducing the peak to average ratio (i.e., crest factor) of energy demandon a utility's system, which in general allows for a more efficientusage of the utility's resources and allows generation to be shifted tothe most efficient production plants available. The demand shift reducesthe average production cost of energy (i.e., average cost of a kW-hour)and may delay or completely eliminate the need to build new powergeneration facilities. Shifting the demand also reduces the impact onthe environment by reducing the carbon emissions.

This system enables an electrical utility to move a significant load (12%-18% ) which is due to domestic water heating from on-peak demandtime windows to off peak time windows, without causing undueinconvenience to the end customer. Because of the advantages, energyproviders may provide incentives for the installation of these systemsby lower energy rates or discount programs. In addition, the system, byvirtue of its energy savings, may qualify for various energy efficiencylabels and government incentive programs, for example Energy Star®.

The approach described here utilizes widely deployed WAN networks(digital cellular, fiber to the home, DSL, broadband over cable, 900mhz, Zigbee, Wi-fi, Wi-max, etc.) to remotely collect data on the enduser's hot water utilizations patterns and current water heater tanktemperatures. The end user's hot water utilization patterns are analyzedby utility server software databases and applications which use thisinformation to segregate customers with similar utilization patternsinto management groups. Based on the aggregate hot water utilizationpatterns of the different management groups, heating of hot water isenabled/disabled via a control downlink over the WAN or localcommunications interface in a manner that minimizes on-peak energyutilization for hot water heating while ensuring that the customer hassufficient hot water to meet their normal daily demands.

FIG. 1 illustrates a system 100 according to one embodiment of thepresent disclosure. Operation of a water heater 101 in a consumer'spremise 108 is controlled by a controller 104. The controller 104communicates to a collar 102 on the premise's meter 103 via a localinterface 106. The collar 102 communicates to a data server 107 via anetwork 105. Operation of the collar 102 is disclosed generally in U.S.Patent Publication No. US2008-0086394, titled “System and Method forControlling a Utility Meter,” which is incorporated herein by reference.The network 105 may be of any type network or multiple networks known inthe art, such as Internet, telephone, Ethernet, analog cellular, digitalcellular, short range radio wireless, Zigbee, HomePlug, Wifi, WiMax,broadband over power line, coaxial cable, and the like. In someembodiments, Wide Area Network (WAN) methodologies will be utilized forcommunicating information and control over the network 105. The network105 may be any combination of hardware, software, or both. The loadmanagement server 107 (also referred to as a control/data server 107) istypically one or more computers adapted to and responsible forcoordinating and controlling the water heaters 101 (i.e., commanding theassociated water heater controller 104 for the water heater 101) andpossibly other appliances and devices on a power grid 110 or portion ofa power grid. The server 107 is in communication with numerous waterheaters 101 on the grid as well as with a database of associated usagepatterns 118 which may be general or specific for each water heater. Theserver is also in communication with power grid controller 114 andgenerating station controller 116 having real time load and capacityinformation as well as expected patterns and other predictioninformation such as weather and planned construction interruptions.Thus, the server 107 may control each water heater 101 to generate acoordinated load that spreads the peak load of the grid system 112 whilemaintaining multiple end customer delivery quality criteria. Thedelivery quality criteria including the availability of hot water invarious quantities ad various temperatures according to individual usepatterns. The server 107 itself may be owned and operated by the companyor utility that owns and operates the power grid 110. Alternatively,multiple organizations and/or agencies may divide the work andresponsibility of operating various parts of the energy delivery 112,114, 116 and load control 107 system.

The local interface 106 is the communications medium between thecontroller 104 and the collar 102. The local interface 106 may be anysuitable communications medium, wired or wireless, standard orproprietary, for example but not limited to: ZigBee, proprietary 900 MHzRF, HomePlug, 802.11a/b/g/n WiFi, or the like.

The system 100 provides for remote monitoring of tank temperature andheating element “on” times and control of the heating elements (eitherdirectly via enable/disable commands or indirectly by adjusting thewater temperature set point). In some embodiments of the system 100, thehardware is retrofitted to existing water heaters 101. This retrofithardware is designed to minimize the labor required for retrofit.

The system 100 comprises of the following: The controller 104 isconnected between the premise's AC mains wiring 109 and the water heater101 AC mains input 110. The controller 104 derives power from the ACmains voltage and can connect/disconnect AC mains voltage to the waterheater 101.

FIG. 2 is an exemplary block diagram of the collar unit of FIG. 1. Thecollar unit 102 performs communication and interface functions for thesystem. The collar unit is aptly named because it may conveniently andpreferably be located in a collar at the base of the electric meter 103;however, other locations may be desired for various reasons. Referringto FIG. 2, the collar unit 102 comprises a processor 202 with a networkinterface 212 for communications with the utility server 107. Theutility server 107 sends commands, demand profiles, algorithms, requestsfor data, and other information to the collar unit 102, which routes thedata or responds as necessary. The collar unit 102 responds to theserver 107 with acknowledgement, status, data logs, and otherinformation as needed. The collar unit 102 may communicate with theserver 107 by one or more communication channels 105 adopted by theutility including telephone, DSL, cable, fiber to the home, digitalcellular, broadband over cable, satellite, 900 MHz, Zigbee, Wi-fi,Wi-max, and others.

The collar 102 also includes one or more interfaces 204-208 for one ormore water heaters 101 and/or other appliances including heating,air-conditioning, freezers, electric car chargers, solar and windgenerators, home automation, security, and others. The interface 106,(generally referenced 106, in FIG. 2 shown as 106 a-106 c to indicatemultiples) may include one or more network media including wired and/orwireless, standard and/or proprietary, and may include Ethernet, Wi-fi,Wimax, Zigbee, Bluetooth, and/or others.

Each of the water heaters controlled by the separate interfaces 204-208may be controlled separately. That is, each may be sent a differentcontrol schedule and/or temperature ranges and each may be individuallymonitored for actual temperature and use patterns. Thus, a kitchen waterheater may see a different use pattern than a bathroom water heater,each offering different load shift opportunities.

The collar 102 may also include an interface 214 to the electric meter103 for electric meter readings 216 including, but not limited to: kWhused, voltage, current, power history logs, outages, etc, and mayinclude connect/disconnect functions for the utility service.

In one embodiment, the collar functionality may be integrated with thecontroller to produce a controller with direct network capability byphone line, cell phone, Wi-Fi or other link that connects with thecontrol server, keeping the network functionality with the water heaterand controller rather than splitting the system as shown in FIG. 1. Infurther integrated solution, the controller and collar functionality maybe integrated into the water heater as a single combined unit. Stillother system partition alternatives are envisioned including having aremote control panel having only user interface features. Multiple otherpartitioning options may be envisioned within the scope of theinvention.

FIG. 3 is an exemplary block diagram of the water heater 101 and thecontroller 104 according to one embodiment of the present disclosure.The temperature of a typical water heater 101 is controlled by an upperthermostat 301, which measures the temperature in the upper portion ofthe water heater's tank, and a lower thermostat 302, which measures thetemperature in the lower portion of the water heater's tank. Intraditional operation of the water heater 101, when the temperature inthe upper portion of the tank drops below a predetermined level, theupper thermostat 301 activates an upper heating element 303. Similarly,when the temperature in the lower portion of the tank drops below apredetermined level, the lower thermostat 302 activates a lower heatingelement 304.

The controller 104 adds the ability to turn off the water heater 101 toreduce load during a peak demand control time interval. The controller104 also controls the heat distribution within the hot water heater 101to maintain hot water availability during the peak demand intervalwithout having to heat the entire tank volume. The controller 104 alsoprovides sequencing and timing of the recovery after the peak demandinterval to prevent a secondary peak demand load from hot water recoveryand to restore full capacity where the capacity is needed most.

The controller 104 comprises a control processor coupled to one or moretemperature sensors. Two temperature sensors are shown in FIG. 3, anupper temperature sensor 307a for sensing the temperature of the uppervolume heated by the upper heating element 303, an a lower temperaturesensor 306 b for sensing the temperature of the volume heated by thelower heating element 304. Since heat rises, the upper heating element303 typically heats only the upper portion of the tank volume, whereas,the lower heating element 304 typically heats the entire tank volume.The control processor 310 turns on the power to the tank 101 usingswitch 313 based on temperature sensors 307 a and 307 b and inaccordance with a temperature specification or profile (temperature vs.time) communicated to the controller 104 from the utility server 107(FIG. 1) via local interface 106. A time of day and calendar clock 314is provided for determination of time of day. The clock 314 may beperiodically set in accordance with standard time provided by theutility load management server 107 over the local interface 106.

The controller 104 may also have voltage inputs for voltage sensingwires 308 a and 308 b connected to the upper heating element 303 andlower heating elements 304 respectively. The voltage sensing wires maybe used to sense the actual operation of the thermostats 301 and 302. Inaddition, the processor may receive inputs from one or more currentsensors 311 and 312 to verify the power being delivered to the hot waterheater 101 via the water heater mains input 110.

In one embodiment of the invention, the upper thermostat may beconfigured with a double throw switch to allow only one heating elementat a time to operate. FIG. 4 illustrates an exemplary water heatercircuit with a double throw thermostat switch. The double throw switchis typically implemented using mechanical bi-metallic switch mechanisms.The double throw switch architecture limits the peak load and reducescircuit requirements for the water heater 101. Referring to FIG. 3 andFIG. 4, Power is applied between terminals 110. The operation is asfollows, starting with a cold water heater 101, the top thermostat 301is switched to the cold side as shown, connecting the top element 303 topower, the bottom element 304 being disconnected from power at switch301. When up to temperature, the top thermostat 301 switches,disconnecting the top element and connecting the lower circuit 402,which includes the lower thermostat 302 and heater 304. Since hot waterrises, the top heating element 303 does not heat the lower water volume.So when the top water volume is heated to temperature, the lower watervolume is still cold. When the upper thermostat 301 switches the powerto the lower circuit 402, the lower thermostat 302 will be in the onstate, which will power the lower heater 304 and heat the remainder ofthe tank contents. When the lower water volume is heated to the lowerthermostat set point temperature, the lower thermostat 302 trips and thewater heater 101 is idle. Incoming water is typically fed into the tankthrough a tube that opens into the tank at the bottom. Thus, as water isused, cold water enters the bottom, tripping the bottom thermostat 302first and turning on the bottom heater 304. If the top water volume everdrops below the top thermostat set point, the top thermostat 301 willswitch, turning off the bottom circuit 402 and feeding power to the topheater 303 for quick recovery, i.e., to heat the smaller upper volumemore quickly than heating the entire volume.

A water heater 101 with a double throw upper thermostat 301 may becontrolled with a controller 104 having a single switch 313 as in FIG.3. The single controller switch 313 works in cooperation with the upperthermostat 301 to control both the upper and lower temperatures during apeak utility demand interval.

During a peak demand interval, the controller 104 turns off power to thewater heater allowing the water heater upper and lower temperatures todrop below the normal maintenance temperature (the lower hysteresispoint for the thermostats), yet maintaining reasonably hot water so thatthe consumer is not without hot water. Thus, the hot water tank ridesout the peak demand interval by drifting down slightly in temperature.The tank 101 may reach and operate at a lower set point temperature thanthat maintained during non-peak intervals. During the interval when thetank temperature is dropping, no power is supplied to the tank heatingelements 303, 304. An idle hot water heater (one not supplying hotwater) should not require any power during the peak demand interval. Ahot water heater that supplies hot water may allow the bottomtemperature to drop to any temperature, but the top temperature istypically maintained at a predetermined minimum temperature throughoutthe peak demand interval. The minimum top temperature is typically belowthe lower hysteresis temperature of the top thermostat 301. For example,the top thermostat 301 may turn on the top element 303 below 130 degreesF. and turn off at 135 F. The corresponding lower minimum temperaturecontrolled by the controller 104 may be 120 F. Thus, a certain amount ofwater may be supplied by the hot water heater 101 during a peak demandinterval without requiring consumption of electrical power by the hotwater heater. When the amount of water supplied is great enough torequire power, only the top quick recovery heating element 303 is used.

Thus, the controller 104 external to the water heater may control thelower temperature to a predefined temperature, defined by the peakdemand interval temperature profile, by supplying power when the upperthermostat switches power to the lower circuit. The switch state of theupper thermostat may be determined by either or both of the voltagesensors with optional confirmation by the current sensors. The topvoltage sensor senses the voltage on the upper heater. If power isapplied by the controller and is not sensed at the upper sensor, theconclusion would be that power is sent to the lower thermostat.Alternatively, voltage may be sensed at the input or output of the lowerthermostat. The input voltage directly indicates the state of the upperthermostat. The output voltage, i.e., sensed by connecting to the lowerheater element as shown in FIG. 3, indicates both the upper and lowerthermostats are switched accordingly.

The temperature of the lower volume may be maintained at a desiredtemperature by observing the switch state of the upper thermostat andapplying power to the lower thermostat to maintain a temperatureaccording to the temperature sensed by the lower temperature sensor.Also, the temperature of the upper volume may be maintained at a desiredtemperature lower than the set point of the upper thermostat bycontrolling power to the water heater when the upper thermostat isconnected to the upper heating element. The power may be appliedaccording to the temperature sensed by the upper temperature sensor.

In addition to voltage sensing, the upper temperature sensor may be usedto determine the upper thermostat switch state. The temperaturemeasurement may be used to determine thermostat switch state when thecontroller has turned off the power to the water heater. Because ofhysteresis in the thermostat switching characteristic, a history of thestate should also be used to track the hysteresis. The thermostat setpoint and hysteresis, i.e., upper and lower switch points may bedetermined by observing the temperature when the voltage monitorsindicate a change of state. Temperature sensors are only a suggestion ofthe switch state. If the temperature sensors indicate the thermostatstate and after application of power, the voltage sensors indicate theopposite, the power may be removed from the water heater and the systemmay wait for additional temperature change and try again.

Various other voltage and current sensing combinations may be used todetermine the state of the upper and/or lower thermostats. In addition,other techniques such as but not limited to special contacts or opticalsensors on the thermostats may also be used to determine the switchstate of the thermostats.

It is a further advantage of several embodiments of the invention, thata controller 104 may be installed in the field to control an existingwater heater 101 which is a commercial item produced to be sold withoutthe water heater controller 104 and having factory installed inputconnection terminals 110. The water heater may be controlled by thecontroller 104 by connecting to the factory installed connectionterminals 110 without interfering with the design of the water heater101 and without making modifications that would or arguably shouldinterfere with a manufacturer's warranty, a safety testing approval, orlocal building codes. Specific warranties and codes may vary and thevarious authorities may disagree, but in principle, the connectionshould not disrupt or bypass any safety feature of the water heater 101.In particular, the original thermostats may be retained and are notmodified. The only control exercised over the water heater is to turnoff power to the water heater as a whole at the external factorysupplied connection point. Temperature sensing may be added withoutdrilling holes or interfering with existing components. Voltage sensewires are added at existing terminal blocks. None of the control andsensing features modify any of the functioning of the original waterheater. Thus, the system should be capable of immediate deployment inlarge numbers without having to replace every water heater on the gridto enable the necessary control.

FIG. 5 illustrates an alternative controller separately controlling eachheating element of a dual element water heater by operating one heaterelement at a time. Referring to FIG. 5, switch 502 operates to turn onand off a selected heating element, and switch 504 operates to selectthe active heating element. Switch 504 cannot power both heatingelements at the same time. Thus, the wire and breaker size in the supplyto the water heater may be sized for a single heating element.Thermostat 301 and 302 are optional and may be used to set a toptemperature above which the associated heating element will be turnedoff. The controller 104 can establish and control to a set pointtemperature for each heater according to the temperature sensors and isnot dependent on the double throw thermostat at position 301 to switchbetween upper and lower heating. The control processor 310 may decidewhich heater to activate at any time. The added control flexibilityallows the control processor to preheat both upper and lower to a higherthan normal temperature just prior to the peak demand control interval,while also allowing the upper volume to act as a quick recovery volumein normal operation at lower temperatures.

The voltage sensing wires 308 a and 308 b may also be optional becausethere is no longer a need to determine the state of the double throwthermostat. If supplied, the wires 308 a and 308 b may be used for faultdetection and/or detection of actual thermostat settings.

FIG. 6 illustrates an alternative controller separately controlling eachheating element of a dual element water heater by operating each heaterindependently of the other.

The system of FIG. 6 includes separate control switches, each separatelycontrolling the upper heating element and lower heating element. Switch602 controls the power to the lower heating element 304 with the returnthrough lead 510. Switch 604 controls power to the top heating element303, with the return through common return lead 510. Current sensors 311and 312 monitor the current through each respective heating element, andvoltage monitor leads 308 a and 308 b monitor the voltage supplied toeach respective heating element. The voltage and current monitors 308 a,308 b, 311, and 312 provide positive verification of the power deliveredto each heating element and verify the state of each of the thermostats.Voltage sensing or current sensing alone may be sufficient. Voltagesensing is simple and very low cost. Current sensing requires a currentsensor, but requires no external wiring and thus, could be costcompetitive with voltage sensing when installation costs are considered.Current sensors have the advantage of allowing the detection of a burnedout heating element.

Although the system control processor and switches 602 and 604 providefull thermostatic control of the water heater 101, the mechanicalthermostats 301 and 302 may be retained as a safety feature to preventoverheating in the event of a stuck relay or other failure. Withsufficient safety testing, however, the thermostats may be removed,relying entirely on the controller 104.

Switches 302 and 304 are shown as single pole switches. Alternativelyeach heating element may be switched using a respective dual pole switchfor each heating element, switching both leads to each heater (thecommon return lead 510 would be split, providing a separate return foreach heating element 303 and 304.)

The increased flexibility of using separate controllable switches foreach heating element allows for greater flexibility in the delivery ofhot water, while avoiding the necessity of heating the entire tankcontents.

One advantage resulting from the increased flexibility is that the waterheater 101 may be operated without using the mechanical thermostats 301and 302 (or by setting the thermostats to a high level) and thus mayestablish the set point to any temperature higher or lower than thenormal temperature and may vary the normal temperature as desired duringthe day or night. Mechanical thermostats define a fixed temperature thatmust be manually changed and thus are impractical for daily control.Mechanical thermostats prevent operation higher than the set point andshould thus be set higher than any anticipated controller commandedoperation. One example where this flexibility is desirable is inproviding a preheat temperature above the normal operating temperature.In the case of a double throw thermostat in the single switch system ofFIG. 3, the quick recovery mode is operated by the thermostat at forexample 135 F/57 C degrees. If it is desired to provide a higher preheattemperature, for example 145 F/63 C degrees, then the thermostat has tobe set higher than 135 F/57 C to allow entire tank heating to 145 F/63C, which defeats the quick recovery mode at 135 F/57 C degrees becausethe double throw thermostat sends power to the lower heater below 145F/63 C degrees. In the system of FIG. 6, however, the thermostats may beset to 145 F/63 C degrees and the controller may command preheat forupper and lower heaters 303, 304 during one interval and may allow quickrecovery using the upper heating element 303 during another interval.

An additional advantage of the configuration of FIG. 6 is that bothheating elements can be operated simultaneously. Normally, the waterheater is designed to use one heater at a time to allow the use ofsmaller circuit breakers and supply wiring. However, if the supplycircuit is sized to operate both heating elements simultaneously bydoubling the supply current capacity, then recovery speed of the fulltank capacity can be increased by operating both heaters simultaneously.Operating both simultaneously potentially aggravates the recovery demandissues after the peak demand interval; however, the double currentdemand may be workable if a small percentage of recovery cycles actuallyuse the feature. For example the double current feature may be triggeredby a limited number of circumstances. Double current may be triggered bydetection of actual water use or by the customer override button orother limited circumstance.

FIG. 6 also shows exemplary optional sensors including an inlettemperature sensor 610, an outlet temperature sensor 608, and a flowrate sensor 606. The sensors may also be applied to the water heaters ofFIG. 3 and FIG. 5. The inlet temperature sensor is shown sensing theinlet pipe and connected to the controller processor 310. A preferredlocation would be a distance from the upper portion of the water heaterto avoid conducted heat from the water heater. Alternatively, the inletwater temperature may be taken to be the lowest temperature measuredover a time interval that includes water usage. The water flowtemperature measurement during usage will avoid conducted heat issues.Outlet temperature may be measured at an outlet pipe. Again,measurements during actual usage will be accurate, but measurementsduring static non use intervals may have errors. A flow rate measuringdevice 606 is also provided as an optional sensor. The flow ratemeasuring device is shown installed in series with the outlet pipe andelectrically connected to the controller processor 310. Alternatively,the flow rate measuring device may be installed to measure the coldwater inlet flow. The inlet temperature sensor 610, outlet temperaturesensor 608, and flow rate sensor 606 may be used to measure deliveredhot water and establish customer use patterns. In addition, as explainedlater in this disclosure, the sensors may be used to determine existingor remaining water heater available hot water capacity at a given pointin the use cycle.

Exemplary nominal specifications for a typical water heater as iscommonly found in a residence are shown in Table 1. Water heaters ofsmaller and larger capacities and with single elements and with elementsof greater or lesser power are often found and may be used with thepresent invention.

TABLE 1 Exemplary Water Heater Specifications Exemplary NominalSpecifications Mechanical thermostat set points settable 110 F./43 C. to150 F./65 C. Typical (exemplary) setting Upper temperature set point forupper 135 F./57 C. thermostat Hysteresis for upper thermostat off,135/57 C. on 130 F./54 C. Lower thermostat set point 135 F./57 C. Lowerthermostat hysteresis off, 135/57 C., on 130 F./54 C. Upper heatingelement 4.5 kw, 240 v Lower heating element 4.5 kw, 240 v Capacity 30 to50 gallons, 120-200 liters

TABLE 2 Exemplary Controller Specifications Typical Programmable, 110 F.to 150 F., 43 to 65 C. (exemplary set point) Holding method programmableHysteresis Programmable 0 F. to 10 F. degrees. 0 C. to 5 C. deg. Minimumdelay 1 to 10 minutes Override switch optional Display Demand controlstatus, Upper temperature, lower temperature, demand time interval,current time, network connection status, collar interface connectionstatus Current monitor optional Voltage monitor Top and bottom, optionalMemory Data log at 1 minute intervals for 1 month

Typical controller specifications 104 may be found in Table 2. Referringto Table 2, the set points for upper and lower (if used) control areaccording to the demand interval command procedure sent to the unit fromthe utility server. The unit 104 should be capable of establishing arange of temperatures. The holding method may be selected. Relays aretypically used with hysteresis or delay to limit chattering.Alternatively semiconductor switches or proportional control may beused. The override switch is optional, but preferably supplied. Thecontroller 104 may have an optional display to display demand controlstatus, upper temperature as measured, lower temperature as measured,the demand time interval in use and the current time, network connectionand collar connection status for debug purposes. The current monitor isoptional. The voltage monitors are optional. At least one voltagemonitor should be used for a water heater with a dual pole thermostat todistinguish the state of the thermostat switch—upper or lower. Thecontroller should have a memory for logging temperature and control datafor a minimum period of, for example one month or other time as deemednecessary.

Any electric residential water heater may be used with the presentinvention, however, a water heater selected or designed for improvedload shedding performance may include additional insulation and mayinclude a larger water capacity.

A single heating element water heater may be used with the presentinvention by operating the single element according to the upper heatingelement procedures, i.e., by lowering the maintenance temperature duringa peak demand interval. In addition, a single heating element waterheater may include an upper and lower temperature sensor and the heatermay be controlled during a peak demand interval according to the uppertemperature sensor, thus allowing hot water delivery (and intake of coolwater) to cool the bottom without turning on the heating element untilthe top begins to cool.

Utility Power Supply/Demand Profile

The peak utility supply and peak consumer demand interval may bealternatively referred to as peak demand interval, or peak demandwindow. The peak demand interval is usually a time interval of maximumpower delivered relative to the daily power delivery cycle. The peakdemand interval may vary from day to day depending on various factorsincluding but not limited to weather and consumer usage patterns.

The peak demand interval or peak demand control interval may also referto an interval established by utility policy either manually orautomatically as the interval within which or with respect to whichdemand spreading measures are to be implemented. The utility powerdemand control interval may be based on historical actual demandprofiles, but may also include the presently active measured demand.

Typical exemplary peak demand times may be five hours, from 4 am to 9 amor from 3 pm to 8 pm. The peak times may be seasonal with the morninginterval being prevalent in the winter, driven by heating and morningshowers and laundry. The summer pattern is driven by air conditioningload and evening water use.

Spring and fall may have less definite peak load times.Weather and predicted weather may influence peak and demand profiles.The peak demand times may be regional due to differing weather patternsand due to differing consumer patterns.

In one embodiment, the peak demand interval for demand spreading may beestablished differently for different groups of customers. Thedifferences among customers may relate to differing usage patterns,differing needs, and/or differing contract charge rates (cents per kwh)for the service. The differing demand interval may relate to the starttime, ending time, or the procedure used to shift demand.

In one embodiment of the invention, the command server may commandindividual water heater controllers by configuring them to runindependently, i.e., the server may establish the procedure once a dayor once a week by providing the time-temperature profile and any otherprocedure decision parameters. The controller then runs independentlyuntil a new command is issued.

In addition, the control server may issue a real time command for aparticular action. For example, the controller may receive informationfrom the grid controller indicating the need for immediate demandreduction. The control server may then issue a command to a large numberof water heaters to turn off immediately, consistent with networkcommunication delays. “Immediate” in this context refers to time periodsshort relative to the segments of a control procedure. Time periods ofless than one minute to tens of minutes are consistent with immediate inthis context. “Real time” and “contemporaneous” refers to interactivelyreading data occurring during a particular control interval and usingthat data to effect a change during the control interval. Thus, thecontrol server may use grid demand measurements for a particular hour toeffect control for that same hour. Thus, the controller and system ofthe present invention allow the server to receive measurements during aparticular day, hour, ten minute span or other interval for use duringthat day, hour, ten minute span, or other interval respectively.

The ability to respond to real time events allows more control thanrelying only on average models. When conditions indicate, water heatersmay be turned down or off to a greater degree or in greater numbers thanwould ordinarily be called for. Thus, greater magnitudes of demandreduction may be achieved. For example, normally all water heaters wouldallow the upper heating element to recover to some level even in demandshift intervals. However, based on real time data, the control servermay turn off all water heaters, both top and bottom elements, for aperiod of time, if deemed appropriate.

This extra level of control may also be used during unexpected events,such as an extreme heat wave, when a generating station or substationfails, or during periods of unexpected emergencies, such as fuelshortages, hurricanes, or other factors limiting peak supply capacity.

The various control temperatures given in this disclosure are exemplarytemperatures. It should be understood that variations from the actualnumbers are easily made by those skilled in the art. In additionvariations from the general characterization of the numbers may also bemade by those skilled in the art.

Normal operating temperature is the temperature of the water in thewater heater during a non-peak demand interval. The normal temperatureis typically desired to be uniform between the upper and lower portionsof the water heater. Thermostat set point temperature is the temperaturemaintained by the top thermostat and may refer to the turn offtemperature or the mid point between the turn on and turn offtemperature.

Minimum operating temperature as discussed with respect to thecontroller refers to the controller enforced set point temperature andis typically established below the thermostat set point temperature.

Quick recovery heating element refers to the top heating element. Thetop heating element heats a smaller quantity of water than the bottomelement and this may recover more quickly from a cold state.

Delivery volume of a water heater typically refers to the upper volumewhich is directly heated by the upper element and the temperature ofwhich is sensed by the upper thermostat and/or temperature sensor. Wateris typically delivered from the top of the water heater.

Inlet volume of a water heater typically refers to the lower volume. Theinlet typically delivers water into the lower volume. The temperature ofthe lower volume is sensed by the lower thermostat and/or lowertemperature sensor.

Hysteresis refers to a technique used to reduce the amount of on and offswitching and thus increase switch contact life. The method is to switchoff at a high temperature, for example, 130 F/54 C degrees, and switchon at a lower temperature, for example 125 F/52 C degrees. The timerequired to heat and cool over the hysteresis range of 5 F/2.5 C degreesreduces contact action and resulting wear on the contacts.Alternatively, a digital controller may utilize a fixed minimum delaybetween changing from on to off or off to on. In a further alternative,with semiconductor switches that have no wear issues, proportionalcontrol by time intervals or voltage levels may be used. Othertechniques may be used as are known in the art.

Customer hot water quality delivery criteria refers to a number ofcriteria for customer satisfaction with respect to the delivery of hotwater. The criteria may refer to one or more of the following exemplarycriteria:

-   -   1) having hot water available at a minimum temperature during        most or all of the day for light usage, such as for sinks;    -   2) having high capacity hot water available during heavy use        periods for a particular customer, heavy usage may include        multiple or long showers;    -   3) maintaining a constant temperature for the delivered hot        water;    -   4) quickness of the recovery from depletion of the hot water        reserve; and/or    -   5) predictability and ease of customer management of hot water        depletion events.

In one embodiment, numerical values may be assigned for deviations fromideal on each of the criteria. Thus, predicted and/or actual performancemay be evaluated using the quality criteria. In a further embodiment,power leveling may be assigned a quality score and load leveling may beweighed automatically with the expected quality to select the amount ofload shift to be delivered.

In accordance with the present invention, the water heater may becontrolled by principles embodied in one or more of the followingexemplary control sequences:

Control Sequence 1

The lower heating element is not turned on during the peak demandinterval. In one embodiment, the power to the water heater is turned offwhen the upper thermostat is off.

Control Sequence 2

FIG. 7 illustrates an exemplary temperature control method forcontrolling an upper and lower heating element of a water heater. Inaccordance with FIG. 7, during a demand control interval, the uppertemperature is reduced, and the lower temperature is reduced or theelement turned off. In one embodiment, upon entering the demand controlinterval, the power to the upper heater is controlled according to theupper temperature sensor in accordance with a set point lower than thenormal operating temperature, and the lower set point temperature islowered, typically lower than the upper set point temperature. In onealternative, the lower element is not used during the peak demandcontrol interval. Referring to FIG. 7, in step 702, the controllerchecks for the current time being within the peak demand interval. Ifnot, step 704 is followed, wherein the water heater is heated to normaltemperatures on the top and on the bottom. If the time is within thepeak demand control interval, step 706 is followed, wherein the top setpoint temperature is dropped from 130 F/54 C degrees to 110 F/43 Cdegrees and the bottom set point temperature is dropped from 130 F/54 Cto 58 F/14 C degrees. Alternatively, in step 706, (not shown) the bottommay be turned off irrespective of temperature. Where the lowertemperature need not be controlled, the lower temperature sensor may notbe needed.

In one embodiment of FIG. 7 using a double throw thermostat, the uppertemperature is controlled by the upper thermostat, i.e., when the upperthermostat calls for upper element heating as sensed by voltage sense308 a (or by lack of voltage on 308 b), then power is applied by relay313. When the upper thermostat switches to the lower circuit as sensedby lack of voltage on 308 a (alternatively by voltage present on 308 b),the control processor then commands relay 313 to power lower heater 304to maintain the desired temperature sensed by lower temperature sensor307 b. Alternatively, the lower heater may not be used during demandshift control intervals. When switch 313 is off, upper and lower voltagesense do not detect a change in state of the upper thermostat, in whichcase, the upper temperature sense 307 a may be used to detect a drop inthe upper temperature sufficient to expect a switch of the upperthermostat and turn on relay 313. If the voltage sense does not thenconfirm the relay has switched to the upper element, then relay 313 maybe turned off and the processor may wait for a further drop intemperature.

Control Sequence 3

In control sequence example 3, the peak demand time interval is dividedinto multiple time segments. A central peak segment is most restrictive.

FIG. 8 illustrates an exemplary multiple segment demand shifting controlsequence. Referring to FIG. 8, the first segment 802 is a pre-peakinterval. During the pre peak interval, in step 804, the water heater ispreheated to a temperature higher than the normal operating temperature,for example 5 F/3 C degrees higher. The higher temperature will allowthe water heater to maintain at least minimum temperature longer and/orto deliver more heat during the peak demand interval without requiringelectrical power.

The second interval 806 is the beginning interval. During the beginninginterval, in step 808, a partial restriction is maintained. For examplethe top set point is lowered to 125 F/52 C degrees and the lower setpoint is lowered to 58 F/14 C degrees.

The third interval 801 is the central peak demand interval. During thecentral demand interval, in step 812, the full restriction isimplemented, for example, the top set point is lowered to 110 F/43 Cdegrees and the lower element is not used, or equivalently, the setpoint is set to zero degrees.

The fourth interval 814, is the end segment. The end segment followsstep 816, which may be less restrictive than the central peak demand,allowing some water heaters to partially recover.

The fifth interval 818 is the post peak demand interval. Post peakdemand interval is in danger of generating a peak of its own by turningon all water heaters at the same instant. Thus the post peak interval818 should include a method of varying the turn on time of the waterheaters. In step 820, the variation may be random, may be by schedule,or may be by a use pattern. An exemplary random pattern may be generatedby having each controller delay the start according to an evenlydistributed pseudorandom delay of zero to 30 minutes. Alternatively, theindividual delay for each controller may be determined by the centralserver 107 and controlled by command by the central server.Alternatively, or in addition to the random delay, the turn on may bedependent on user patterns or on actual tank temperatures or tankremaining capacity. For example, cold lower tank at the end of the peakdemand interval may indicate actual water use and may the water heatermay merit earlier complete recovery, whereas an untouched tank showingonly nominal temperature decay through normal cooling, may indicate noone home using or needing hot water and thus may be further delayedwithout consumer impact. Normal usage patterns may also be consideredwith respect to delaying water heater recovery.

The recovery itself is staged 824 so that the top quick recovery volumeis heated first followed by the lower tank volume.

In step 826, after reaching normal temperature, normal temperature ismaintained 828 until the next peak demand interval.

Each of the above segments are exemplary and optional. Many differentdemand control profiles may be generated by varying the above exemplarysegments and temperature and time parameters in accordance with theteaching herein.

Power Failure

The system may also mitigate the power surge upon restoring power aftera power failure. By designing the power on characteristic of thecontroller to hold the power relays off for a few seconds until thecomputer is operational and able to assess the situation, the waterheater will not present a load during the first few seconds whereutility power is used to start refrigerators and other large motors.Once the computer is operational, the computer may contact the serverfor instructions, and/or may implement a turn on delay process 820before turning the water heater back on. Thus, the controller may delaythe power turn on to reduce both the immediate spike as well as a longerdemand peak as water heaters are recharged.

Volumetric Usage and Capacity

In accordance with on embodiment of the invention, volume of hot waterusage in gallons or liters at a desired temperature are estimated bymeasuring the upper temperature and lower temperature and observing thepower input to the water heater. Referring to FIG. 1, the power input tothe water heater may be determined by knowing the voltage at thecustomer's location and the resistance of the water heater element. Thetime interval that the water heater is drawing power may be determinedby observing the current sensor, if so equipped, or by measuring thevoltage on the heating element, if the voltage sense wire is installed.The relay power alone may not be sufficient for water heaters having amechanical thermostat; however, either the voltage sense or currentsense can eliminate ambiguity in the thermostat state. Thus, the powerinput over time can be monitored.

Usage can be monitored by installing a flow measurement device at thewater heater. Gallons or gallons per minute of usage can be recorded bythe processor and an average usage can be determined over time. Forexample, usage can be recorded each day for each ten minute interval. Arunning average may be determined for each respective ten minuteinterval for the last 30 days. Other daily intervals may be used andother numbers of days for the running average may be used. Thus, anaverage expected demand as a function of time during the day may begenerated for each water heater. Further, data from weekends and orholidays may be separated from week days to further refine theestimates, e.g., data from the last 10 week days may generate a weekdayrunning average and data from the last 10 weekend days may generate aweekend running average. Thus, the average daily pattern may becontinuously updated on a periodic (daily) interval.

Alternatively, usage may be estimated by monitoring the upper and lowertemperature sensors 608, 610 in combination with monitoring the powerinput to the water heater 101. A model of the vertical temperatureprofile of the water heater is developed and used to solve for the usagethat results in the measured temperatures at the upper and lowerlocations. The power input to the upper and lower heater elements isdetermined from voltage and/or current monitors. The water inlettemperature may be measured or may be estimated. A temperature sensor610 may be installed at the inlet pipe serving the water heater. In theabsence of a direct measurement, the inlet water temperature may beestimated as the typical underground temperature in the area for theseason. The utility may sample a few locations and use the temperaturefor all water heaters. Alternatively, the inlet temperature may beestimated as the lowest temperature achieved by the lower temperaturesensor during a long power off cycle. The lower temperature willasymptotically approach the inlet temperature in the absence of powerinput when there is water usage.

A simple water heater vertical temperature profile may be implemented asa linear temperature gradient from the location of the full heatedtemperature to the inlet temperature. Alternatively, other curves suchas an exponential or logarithmic curve may be used. Ideally, measureddata may be taken from several water heater types and used to generateempirical models of the water heater vertical temperature profile as afunction of usage and power input.

FIG. 9 is a notional depiction of a typical water heater verticaltemperature profile model as hot water is used from the tank. FIG. 9depicts the concept generally, but does not show measured values.Referring to FIG. 9, the temperature is plotted as might be measured onthe side of the tank at the location shown on the X axis. Plots 902-908show different increasing levels of usage of the hot water in the tankwithout the addition of power to restore the tank to full temperature.Each graph 901-908 may represent, for example 10 additional gallons ofusage. The upper temperature probe 910 and lower temperature probe 912locations are shown. Initially 901, the tank is heated to fulltemperature and the tank equalizes through conduction of heat. As hotwater is used, cool water enters the bottom of the tank 902. As coolwater continues to fill the bottom of the tank, the cool level rises andsome mixing distributes the cool water vertically 904. The processcontinues to 906 and 908, at which point the upper temperature sensorreaches the control level and the upper element is turned on to maintainthe upper temperature. A sequence of curves 901-908 may be used as amodel of the progressive usage of water from the tank. Alternatively, apiecewise linear approximation, such as plot 914 may be used. Plot 914approximates plot 906. A series of plots like 914 would approximate901-908.

In operation, the temperature at location 912 and 910 would be used todetermine the plot 901-908 that is closest to the measured temperatures,and the present usage and remaining capacity may then be determined.Interpolation may refine the estimate. Normally, the water heater tankis depleted without adding power during the demand shift period and thenfully restored to plot 901 after the demand shift period. If heat ispartially restored as demand resumes, additional families of graphs maybe used to refine the estimate.

In addition to the usage and capacity obtained by monitoring the tanktemperatures, overall usage volume may be obtained by computing volumebased on delivered temperature, input temperature, and power consumed.The overall usage may be computed without modeling the vertical profile.The input temperature is estimated as above, and the output temperatureis measured or estimated. In the absence of a measurement at the outputpipe, the upper temperature probe or thermostat setting may be used asan output estimate. The delivered volume is then:

$V = \frac{Pt}{\rho \; {c\left( {T_{out} - T_{in}} \right)}}$

-   -   where,    -   V is the volume, e.g. ml.    -   ρ is the density of water, e.g. 1 g/ml    -   c is the specific heat capacity of water, e.g., 4.18 Jg⁻¹K⁻¹    -   P is the power delivered to the water heater, e.g., watts,    -   t is the total time either the top or bottom heater is on, e.g.        seconds,    -   T_(out) is the outlet temperature of the water heater, e.g. C,    -   T_(in) is the inlet temperature to the water heater, e.g. C.        The total volume delivered should be computed between time        points having the same storage state, e.g., plot 901, fully        heated, to avoid errors due to different stored heat values.

Peak Demand Load Shift

According to one embodiment, baseline demand and baseline usage patternsare obtained without commanding any control on the water heaters, i.e.,by operating all water heaters at full regular temperature, both top andbottom heaters. A control strategy is devised and control processes aredownloaded to each water heater controller. The control is operated fora period of time and the controlled demand and usage patterns are thenmeasured. The baseline and controlled patterns are compared to determinethe change as a result of the control. Periodically, or as needed, thecontrol may be turned off to reestablish the baseline. Baseline andcontrol time periods may be preferably from one to two weeks to gathersufficient data. Shorter and longer periods may also be useful dependingon the data being gathered.

Customer Use Patterns

In accordance with one embodiment of the invention, customer usepatterns may be determined and used to select water heaters to be usedfor load shifting. Patterns may be determined on a continuously orperiodically updating basis so that changes in customer status arereflected in the current database. In addition to expected patterns,control may be based on the actual measured capacity and actual measureduse. For example, the historical use pattern may indicate a fully heatedwater heater is expected at customer A, but current measurementsindicate only 30% remaining capacity with ongoing use. Thus, customer Awould not be a candidate for load shift at the current moment and thesystem would move on to select a different customer.

By detecting water usage patterns, the control server may identify thelocation of each of several water heaters within a household as beingprimarily kitchen, bath, or guest room use and may classify each inassociated groups with other households and may control each separately.

According to one embodiment, the load is shifted by identifying waterheaters that have a near term measured historical use pattern (dailytime period) outside of an expected peak demand period where the waterheater may be turned down or off during the peak demand period andturned back on after the peak demand period to return to normaltemperature before predicted use of the water heater. For example, awater heater may have a low probability of use between 3:00 PM and 5:00PM. The water heater may be selected for load shifting to mitigate ananticipated 4:00 PM to 5:00 PM peak.

According to another embodiment, water heaters may be grouped accordingto use patterns and operated as groups. For example a first group mayhave a use pattern showing low probability of use between 3:00 PM and5:00 PM. A second group may show a low probability of use between 3:00PM and 6:00 PM and so on. Both groups may be selected to mitigate a 4:00PM to 5:00 PM peak. The first group may then be the first to recover tofull temperature after the 4:00 PM to 5:00 PM peak.

According to another embodiment, water heaters may be grouped accordingto the primary function or associated room being supplied by the waterheater, such as kitchen, bathroom, guest room, whole house, or other.The group may be by measured pattern or by a survey form entered by theinstaller or an inspector. The kitchen may have a typical use pattern of0.2 gallons (0.8 liters) per minute and may include midday use. Thebathroom may include showers at 2 gallons (8 liters) per minute withprimary use morning and night. A guest room may have infrequent usage.Numerous other patterns and categories may be identified.

A household may be classified according to the number of adults andchildren served by the water heater.

According to another embodiment, a reserve capacity may be establishedfor a water heater and under normal circumstances, the water heater isnot allowed to go below the reserve capacity without restoring power tothe water heater. Water heaters may be identified and/or groupedaccording to the probability of use beyond the reserve capacity.

According to another embodiment, the water heater (lower element) may beturned down or off until a period just before predicted use at whichpoint, the water heater is turned on in order to recover to normaltemperature by the beginning of the predicted use time.

According to another embodiment, if the predicted use time is during ademand peak, the water heater is heated to full temperature before thedemand peak, turned off during the demand peak until actual measuredusage dictates more hot water is needed and then water heater power isresumed during the demand peak to supply the actual usage.

According to another embodiment, a water heater which is turned down ona long term basis due to infrequent usage may be turned up somewhatbefore a demand peak to avoid a thermal maintenance (hysteresisrecovery) cycle during the demand peak and then turned up later inanticipation of a predicted usage period.

According to another embodiment, users who historically demand hot waterfirst are powered first to recover to full capacity after the peakrecovery interval begins.

According to another embodiment, users may periodically report theirtemperatures and users with the lowest temperatures may be given powerfirst.

Retrofit of Existing Water Heaters

New water heaters may be designed in accordance with the presentinvention by incorporating a controller into the water heater and/or byintegrating water heater thermostat sensing and control functions withthe controller.

Existing water heaters need certain modifications to provide the sensingneeded by the controller. Depending on the controller featuresimplemented, top or bottom or both temperature sensors may be installedand top or bottom or both voltage sensors may be installed. The toptemperature sensor may preferably be added to the mounting for the topthermostat as this mounting is designed to have proper thermal couplingto the water in view of the convection currents caused by the heatingelement. Alternatively, some suitable locations may be found on the sideof the water heater. The top sensor should generally be higher than thelower sensor and should measure temperature related to the upper heatingelement heating. The temperature sensors may be attached by adhesive,such as epoxy, or by wedging the sensor between the tank and outer caseof the water heater. Other attachment techniques may be used. Apreferred attachment device for the thermostat location is nowdescribed.

In order to retrofit an existing water heater 101 for the purposes ofthe present invention, two temperature sensors, an upper temperaturesensor 305 and a lower temperature sensor 306, are installed at the topand bottom thermostats 301 and 302 of the water heater. Mechanically,each of these temperature sensors 305 and 306 include a thin metal plateof approximately the same footprint dimensions as the existingthermostats 301 and 302. At the top of these metal plates is a tab (notshown) with a temperature sensor attached (i.e. a J-K thermocouple or asemiconductor type temperature sensor). These metal plates are installedbetween the existing thermostats and the inner wall of the water heatertank. Since these plates are thin (typically 1/16^(th) inch or less (1.5mm)) and the metal from which they are made is a good thermal conductor,it is expected that there will be a very small temperature drop acrossthe plate (i.e. from the tank wall to the back of the thermostat) sothat the insertion of this temperature sensor has have a negligibleimpact on the operation of the existing thermostats. Each of these twotemperature sensors 305 and 306 typically has a low voltage wire pair307 a and 307 b that connects back to the controller 104 so that theupper and lower tank temperature may be measured and reported by thecontroller 104.

In some embodiments, voltage to each of the heating elements 303 and 304is measured via a pair of wires 308 a and 308 b connected across theupper and lower heating elements. Each of these wire pairs 208terminates back at the controller 104 so that a determination may bemade as to which heating elements (i.e. upper 303 and/or lower 304) arecurrently energized. It may be necessary to put either a fuse or highimpedance resistor in series with the wire pairs 308 a and 208 b at theorigination point so as to eliminate any high voltage electrical hazard.

Current sensors 311 and 312 on the AC mains inside the controller 104are used to determine the time of use energy consumption of the entirewater heater 101. The current sensors 311 and 312 are also useful as adiagnostic aid to determine when a heating element (303 or 204) orthermostat (301 or 302) has failed (e.g. an energized heating elementthat is not drawing current is presumably failed open). Although FIG. 3illustrates two (2) current sensors 311 and 312, some embodiments of thesystem use only one (1) current sensor. Two sensors are not necessarilyrequired because the current in each leg should be the same.

Installation of the Temperature Sensors 305 and 306

The sensor wires may be installed by unscrewing the lugs for AC mainswiring connected to the bottom heating element and any clamps holding itin place. A new AC mains wire set of sufficient capacity and type (i.e.complying with local and national electrical code), a low voltage sensorwire pair (which is connected to the temperature sensor plate 305 or306), and a voltage sensor wire pair which will be connected in parallelwith the bottom heating element are assembled as a bundle. This may bepractically accomplished by using a pre-made wiring harness. Theresulting wire bundle is temporarily connected to the original lowerelement AC mains wiring. The original AC mains wire is pulled from theupped thermostat opening, pulling the new wire bundle to the topthermostat opening. The new AC mains wiring is reconnected in place ofthe original AC mains wiring to the lower element. The lower thermostatis removed, and the lower temperature sensor plate is installed on theback of the original lower thermostat and the thermostat is replaced.The voltage sensor wire pair is connected to the lugs which carryvoltage to the lower heating element. The lower thermostat cover isreplaced. At the upper thermostat opening, the original thermostat isremoved and the upper temperature sensor plate is installed on thebackside of the thermostat which is then replaced. The upper heatingelement voltage sensor wire pair is connected across the upper heatingelement. All of the wiring from the upper and lower thermostats/heatingelements are connected to the designated terminals on the control unit.Installation should take only about 30-45 minutes per installation.

Override Switch

In one embodiment the controller may have a customer option to overridethe water heater for the customer to turn on regardless of the peakingconditions. Use by customers of the override option should be rare sincethe customer's behavior would have already been studied and the customershould always have hot water. In the event that the override switch isused, customer will be billed at the premium rate based on the timeassociated with the usage.

The customer override function is activated by a switch or button on thewater heater controller. In the event the customer activates theoverride function, a notification will be sent to the server and theserver will deactivate the override after a predetermined time periodhas passed.

Water Heaters with a Factory Installed Temperature Probe.

The water heater controller unit will operate with newer models of waterheaters that have a temperature probe installed at the factory, byutilizing the factory installed probe sensors to connect to thecontroller unit.

Conclusion

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed. Any such alternate boundaries are thus within the scope andspirit of the claimed invention. One skilled in the art will recognizethat these functional building blocks can be implemented by discretecomponents, application specific integrated circuits, processorsexecuting appropriate software and the like or any combination thereof.

Specific applications have been presented solely for purposes ofillustration to aid the reader in understanding a few of the great manycontexts in which the present invention will prove useful. It shouldalso be understood that, while the detailed drawings and specificexamples given describe preferred embodiments of the invention, they arefor purposes of illustration only, that the system and method of thepresent invention are not limited to the precise details and conditionsdisclosed and that various changes may be made therein without departingfrom the spirit of the invention which is defined by the followingclaims:

1. A water heater controller for controlling a water heater to alterelectrical demand while providing hot water for normal use, said waterheater comprising an upper heater for heating an upper volume of waterand a lower heater for heating a lower volume of water; said waterheater controller comprising: a power control means for varying thepower supplied to said upper heater and for separately varying the powersupplied to said lower heater; a processor having a memory, said memorystoring a load shift algorithm comprising instructions for varying powerapplied by said power control, said load shift algorithm specifying areserve volume capacity for said water heater, a lower temperaturesensor measuring a temperature of said lower volume of water andproviding a resulting lower volume temperature measurement to saidprocessor; means for determining a remaining volume capacity of hotwater in said hot water heater; wherein said processor operates saidpower control device to control a temperature of said lower volume ofsaid water heater in accordance with said load shift algorithm when saidremaining capacity exceeds said reserve capacity.
 2. The water heatercontroller of claim 1, further including a network interface, whereinsaid processor receives at least a part of said load shift algorithmover said network interface from a control server, said control serverin communication with a plurality of water heater controllers atseparate customer locations on a power grid.
 3. The water heatercontroller of claim 2, wherein said processor provides said remainingvolume capacity determination to said control server and said controlserver selects said water heater for load shift control in accordancewith said remaining capacity of said water heater.
 4. The hot waterheater controller of claim 2, wherein the processor records historicalusage data comprising temperature measurements and power control, andsaid processor provides said historical usage data to said controlserver over said network interface and said control server generatessaid load shift algorithm based on said historical usage data.
 5. Thehot water heater controller of claim 1, wherein the water heaterincludes at least one mechanical thermostat, and further including avoltage sensor sensing the voltage at an output of said at least onemechanical thermostat to determine a switch state of said at least onemechanical thermostat.
 6. The hot water heater controller of claim 1,wherein the water heater controller further receives a command from saidcontrol server to override the requirement for when said remainingcapacity exceeds said reserve capacity, and to lower a temperature ofsaid water heater regardless of the remaining capacity.
 7. The hot waterheater controller of claim 1, wherein the water heater controllerfurther receives a command from said control server to override therequirement for when said remaining capacity exceeds said reservecapacity, and to turn off power to the upper and lower heater regardlessof the remaining capacity.
 8. A method for controlling a plurality ofwater heaters to reduce peak loading on a power grid comprising thesteps of: receiving temperature data at a water heater control serverfrom a plurality of water heaters connected to said power grid atseparate customer locations, generating usage pattern information foreach water heater based on said temperature data; receiving load demandinformation at said water heater control server from a power gridmonitoring system monitoring said power grid; said water heater controlserver commanding a control strategy comprising a temperature limit overtime for each water heater of said plurality of water heaters based onsaid received temperature data, said load demand data, predicted loaddemand information; wherein each water heater of said plurality of waterheaters is controlled in accordance with hot water delivery qualitycriteria applied to said usage pattern information specific to each saidwater heater.
 9. The method of claim 8, wherein the water heater controlserver commands a control procedure that includes allowing a temperatureof a delivery volume of a water heater to drop to a lower temperaturethan normal operating temperature to delay water heater electricaldemand.
 10. The method of claim 8, wherein the water heater controlserver commands a control procedure that includes allowing a temperatureof an inlet volume of a water heater to drop to a lower temperature thannormal operating temperature to delay water heater electrical demand.11. The method of claim 8, further including the step of: receiving realtime peak demand information from said power grid monitoring system;sending a command by said water heater control server to a majority ofsaid plurality of said water heaters to reduce demand in response tosaid peak demand information reported by said power grid monitoringsystem.
 12. A method for shifting a load of a power grid comprising thesteps of: providing a control server in network communication with aplurality of water heater controllers controlling a respective pluralityof water heaters; determining historical water heater usage patterns forsaid plurality of water heaters; said historical usage patternscomprising an expected usage as a function of time over at least onedaily interval; receiving a load shift goal at said control server froma power grid controller for said power grid; selecting a first set ofwater heaters of said plurality of water heaters based on saidhistorical water heater usage patterns; receiving contemporaneous hotwater remaining capacity measurements by said control server for eachwater heater of said first set of water heaters; selecting a second setof water heaters based on said contemporaneous hot water remainingcapacity measurements, said second set of water heaters being a subsetof said first set of water heaters; commanding said respective waterheater controllers for said second set of water heaters to reduce atemperature associated with a lower heating element of each water heaterof said second set of water heaters.
 13. The method of claim 12, whereinthe step of selecting said first set of water heaters includes selectingwater heaters in accordance with at least one group, the members of saidat least one group having at least one common usage category attribute.14. The method of claim 12, wherein the remaining capacity is based on avolume measurement of hot water.
 15. The method of claim 14, wherein thevolume measurement is based on an upper temperature measurement proximalto an upper heating element and a lower temperature measurement proximalto a lower heating element together with power delivered to the waterheater.
 16. The method of claim 14, wherein the volume measurement ofhot water is based on at least one temperature measurement between thetop and the bottom of the water heater and a heat distribution model ofthe hot water heater vertical temperature profile as a functioncomprising usage and power applied to the water heater.
 17. The methodof claim 12, wherein the step of selecting by historical patternsincludes selecting at least one water heater according to expectedfuture demand.
 18. The method of claim 17, wherein the expected futuredemand is the greatest time interval in the future.
 19. The method ofclaim 12, wherein the usage is based on a volume measurement of hotwater.
 20. The method of claim 19, wherein the volume measurement isbased on an upper temperature measurement proximal to an upper heatingelement and a lower temperature measurement proximal to a lower heatingelement together with power delivered to the water heater.
 21. Themethod of claim 19, wherein the volume measurement of hot water is basedon at least one temperature measurement between the top and the bottomof the water heater and a heat distribution model of the hot waterheater vertical temperature profile as a function comprising usage andpower applied to the water heater.
 22. The method of claim 12, whereinthe second set of water heaters is further selected based on maintaininga minimum available remaining capacity at each water heater of saidsecond set of water heaters.
 23. The method of claim 12, wherein anaverage daily usage pattern is determined as an average volumetric usagerate as a function of time over a 24 hour interval.
 24. The method ofclaim 23, wherein the average daily pattern is determined as a movingaverage of a predetermined number of daily patterns determined for eachday of said predetermined number of days.
 25. A water heater controllerfor controlling a water heater to alter electrical demand, said waterheater comprising an upper heater for heating an upper volume of waterin accordance with an upper thermostat and a lower heater for heating alower volume of water in accordance with a lower thermostat, said upperthermostat switching from said upper heater to said lower thermostatwhen said upper volume of water is above an upper thermostat set point;said water heater controller comprising: a power input for receivingpower from a power source; a power output for connecting to said waterheater and supplying power to said water heater; a power control deviceconnected between said power input and said power output for varying thepower supplied to said water heater; a processor having a memory, saidmemory storing a load shift algorithm for varying power applied to saidwater heater to alter the electrical demand by said water heater, saidprocessor controlling said water heater by controlling said powercontrol device; a lower temperature sensor measuring a temperature ofsaid lower volume of water and providing a resulting lower volumetemperature measurement to said processor; a thermostat state sensordetermining a switch state of said upper thermostat and communicatingsaid switch state to said processor; wherein said processor operatessaid power control device to control a temperature of said lower volumeof water when said upper thermostat is switched to deliver power to saidlower thermostat, said temperature of said lower volume of watercontrolled in accordance with said load shift algorithm, said lowervolume temperature measurement, and said switch state of said upperthermostat.
 26. The water heater controller of claim 25, wherein theupper thermostat state sensor comprises a voltage sensing associatedwith said upper thermostat.
 27. The water heater controller of claim 25,wherein the water heater is a commercial item produced to be soldwithout said water heater controller and having a factory supplied powerinput terminals and said water heater controller controls said waterheater through said factory supplied power input terminals.
 28. Thewater heater controller of claim 27, wherein the water heater controlleris installed to control said water heater without bypassing safetyrelated components on said water heater.
 29. The water heater controllerof claim 25, further including an upper volume temperature sensor incommunication with said processor for producing an upper volumetemperature measurement, and said lower volume of water is furthercontrolled in response to said upper volume temperature measurement. 30.The water heater controller of claim 29, wherein the processor uses theupper volume temperature measurement to determine whether the upperthermostat is switched to the lower thermostat when the power controlleris switched off.